ABSTRACT. Compared to normal tissues, there are well documented and functionally important changes in mechanical properties of neoplastic cells and tumor tissue, including tissue stiffening, loss of elasticity, and densification. Clinically, high tumor stiffness correlates with aggressive subtypes of epithelial cancers and overall poor prognosis. The importance of mechanical deregulation is illustrated by evidence that cancer cells increase their metastatic potential when cultured on substrates closely resembling stiffness of tumor microenvironment. Mechanistically, a self-reinforcing link between cancer cell and ECM remodeling likely results in increased stiffness and progressive malignization of tumor cells. Despite a growing appreciation in the importance of understanding mechanical signaling in tumor biology, the methods for reliable high resolution measurements of tumor mechanical properties in 3D cell models and live tumors are severely lacking. Therefore, there is an important need to overcome this technological limitation in order to advance our understanding of cancer progression and therapeutic strategies to revert this process. Here we address this limitation by developing a new high-spatial resolution method for nanobomb-Optical Coherence Elastography (nb-OCE). OCE is an emerging optical non-invasive biomechanical imaging method that, in principle, can detect tissue stiffness with micrometer resolution. However, existing excitation and detection methodologies for force measurements are limited in their ability to produce highly localized mechanical stress that is needed for high-resolution 3D elastography mapping. To solve this limitation we propose novel ?nanobomb? contrast agents for OCE that are based on lipid-coated perfluorocarbon (PFC) nanodroplets with embedded light absorbing dyes. Illumination of PFC ?nanobombs? by a pulsed laser triggers liquid to gas transition of PFC nanodroplets due to heating produced by light absorbing chromophores in the PFC core. Our preliminary data showed that this liquid-gas phase transition induces highly localized mechanical stress that can be detected by OCE with a high signal-to-noise ratio (SNR). Further, this ?bursting? of PFC nanobombs and their expansion can be effectively triggered by a femtosecond laser allowing a straightforward combination of the proposed here nb-OCE and multi-photon microscopy (MPM). This combination will provide an unprecedented opportunity to measure tissue elasticity with high resolution in the context of tissue morphology and function given by MPM. Here we assembled a team of experts in nanotechnology (Sokolov), OCE instrumentation/data analysis (Larin) and multiphoton intravital imaging/tumor biology (Friedl) to demonstrate feasibility of this combined technology in 3D spheroid cell cultures in vitro and in window tumor models in small animals. This MPM-nbOCE technology will enable longitudinal monitoring of evolution of tissue elasticity landscape during both tumor growth and invasion thus addressing a critical gap in our understanding of tumor biology.